Batteries with lithium metal anodes represent the highest energy density of any commercially available batteries. While the redox reactions that liberate the energy from the lithium metal chemistry are reversible, rechargeable batteries with lithium metal anodes are not commercially available. Such rechargeable batteries are not commercially available because of the potential of fire and explosion caused by the growth of lithium dendrites during the recharge cycle. The recharge cycle can create a short circuit directly between the anode and cathode. Several strategies for preventing shorting dendrites have been advanced with varying degrees of success, but none have yet reached the level of safety required for commercial acceptance. Indeed, eliminating the potential for dendritic growth remains a major consideration in the design of lithium ion batteries, which are much less susceptible to the phenomenon. Among the most promising approaches for reducing the fire and explosion risk in advanced LIBs (Lithium Ion Batteries) with lithium metal anodes is the use of solid-state, ceramic electrolyte as the separator between anode and cathode. Use of ceramic separators not only virtually eliminates the risk of dendrite growth but also removes all flammable material from the cell. Thus, even if a short circuit does occur, there is nothing in the cell to burn or explode. An additional advantage of ceramic electrolyte is its inherent stability at voltages well beyond the stability limits for conventional liquid electrolyte. Conventional electrolyte breaks down when the voltage difference between anode and cathode is much above 4 volts. Ceramic electrolyte display electrochemical stabilities well beyond six volts. Increasing the cell voltage from 4 volts to 6 volts increases the energy of the cell by 50%. Ceramic electrolyte also confers significant advantages in the volume of materials used for cell packaging. For example, the ceramic electrolyte does not outgas like a liquid electrolyte.
The concept of lithium based batteries with ceramic electrolytes has been around for many years, and many cells employing thin film fabrication techniques have been manufactured. However, these cells suffer from very low specific energy and very high cost. One of the major problems with previous efforts at practical cells with ceramic electrolytes stems from the properties of the ceramics themselves. All the ceramic materials recognized as practical candidates for application as solid-state electrolytes are very hard and brittle, and they must be very thin to act as practical separators in a lithium ion cell. The requirement for thinness comes both from the need to limit the volume of materials, such as electrolyte, that do not directly contribute to the storage of energy. Furthermore, electrolyte volume can be limited because the best of ceramic electrolytes are still not great conductors of lithium ions and must be as thin as possible to limit the internal resistance of the cell. Typically, experts in the field of solid-state batteries consider that the best ceramic electrolyte separators should be no more than 40 μm thick. Producing such thin flat sheets of brittle ceramic by conventional means is difficult and expensive. Moreover, the end product is very difficult to incorporate into a volume manufacturing scheme. Thin flat sheets of brittle ceramic are also very susceptible to fracture when the layers of the cell are pressed together to assure adequate contact among the various layers of the cell.